Lecture 3b— Introduction to semiconductors

ECE 4833 - Dr. Alan DoolittleGeorgia Tech

Lecture 3

Introduction to Semiconductors and Energy Bandgaps


Solar Cells

Why do the electrons flow when light is present but not flow when light is not present?

Answer, Energy Bandgap (very important concept).


•For metals, the electrons can jump from the valence orbits (outermost core energy levels of the atom) to any position within the crystal (free to move throughout the crystal) with no “extra energy needed to be supplied”•For insulators, it is VERY DIFFICULT for the electrons to jump from the valence orbits and requires a huge amount of energy to “free the electron” from the atomic core.•For semiconductors, the electrons can jump from the valence orbits but does require a small amount of energy to “free the electron” from the atomic core.

Classifications of Electronic Materials


•Semiconductor materials are a sub-class of materials distinguished by the existence of a range of disallowed energies between the energies of the valence electrons (outermost core electrons) and the energies of electrons free to move throughout the material.•The energy difference (energy gap or bandgap) between the states in which the electron is bound to the atom and when it is free to conduct throughout the crystal is related to the bonding strength of the material, it’s density, the degree of ionicity of the bond, and the chemistry related to the valence of bonding.•High bond strength materials (diamond, SiC, AlN, GaN etc...) tend to have large energy bandgaps.•Lower bond strength materials (Si, Ge, etc...) tend to have smaller energy bandgaps.




•More formally, the energy gap is derived from the Pauli exclusion principle, where no two electrons occupying the same space, can have the same energy. Thus, as atoms are brought closer towards one another and begin to bond together, their energy levels must split into bands of discrete levels so closely spaced in energy, they can be considered a continuum of allowed energy. •Strongly bonded materials tend to have small interatomic distances between atoms. Thus, the strongly bonded materials can have larger energy bandgaps than do weakly bonded materials.


Material Classifications based on Bonding MethodBonds can be classified as metallic, Ionic, Covalent, and van der Waals.


Consider the case of the group 4 elements, all** covalently bonded

Element Atomic Radius/Lattice Constant Bandgap(How closely spaced are the atoms?)

C 0.91/3.56 Angstroms 5.47 eV

Si 1.46/5.43 Angstroms 1.12 eV

Ge 1.52/5.65 Angstroms 0.66 eV

-Sn 1.72/6.49 Angstroms ~0.08 eV*

Pb 1.81/** Angstroms Metal

*Only has a measurable bandgap near 0K

**Different bonding/Crystal Structure due to unfilled higher orbital states


Classifications of Electronic MaterialsTypes of Semiconductors:•Elemental: Silicon or Germanium (Si or Ge)•Compound: Gallium Arsenide (GaAs), Indium Phosphide (InP), Silicon Carbide (SiC), CdS and many others

•Note that the sum of the valence adds to 8, a complete outer shell. I.E. 4+4, 3+5, 2+6, etc...


Compound Semiconductors: Offer high performance (optical characteristics, higher frequency, higher power) than elemental semiconductors and greater device design flexibility due to mixing of materials.

Binary: GaAs, SiC, etc...

Ternary: AlxGa1-xAs, InxGa1-xN where 0<=x<=1

Quaternary: InxGa1-xAsyP1-y where 0<=x<=1 and 0<=y<=1

Half the total number of atoms must come from group III (Column III) and the other half the atoms must come from group V (Column V) (or more precisely, IV/IV , III/V, or II/VI combinations) leading to the above “reduced semiconductor notation.

Example: Assume a compound semiconductor has 25% “atomic” concentrations of Ga, 25% “atomic” In and 50% “atomic” of N. The chemical formula would be:

Ga0.25In0.25N0.5

But the correct reduced semiconductor formula would be:

Ga0.5In0.5N



Material Classifications based on Crystal Structure

Amorphous MaterialsNo discernible long range atomic order (no detectable crystal structure). Examples are silicon

dioxide (SiO2), amorphous-Si, silicon nitride (Si3N4), and others. Though usually thought of as less perfect than crystalline materials, this class of materials is extremely useful.Polycrystalline Materials

Material consisting of several “domains” of crystalline material. Each domain can be oriented differently than other domains. However, within a single domain, the material is crystalline. The size of the domains may range from cubic nanometers to several cubic centimeters. Many semiconductors are polycrystalline as are most metals.

Crystalline MaterialsCrystalline materials are characterized by an atomic symmetry that repeats spatially. The shape of

the unit cell depends on the bonding of the material. The most common unit cell structures are diamond, zincblende (a derivative of the diamond structure), hexagonal, and rock salt (simple cubic).


Increasing structural quality and increasing efficiency (performance)


Chemical Bonding Determines the

Bandgap


Comparison of the Hydrogen Atom and Silicon Atom

Hydrogen

Silicon

...3,2,1,arg2/2/tan,

6.13)4(2 22

0

4

nandechelectronqhtconsplanksmasselectronmwhere

neV

nqmEnergy

o

oelectronHydrogen

n=1: Complete Shell2 “s electrons”

n=2: Complete Shell2 “2s electrons”6 “2p electrons”

n=3:2 “3s electrons”Only 2 of 6 “3p electrons”

4 empty states

4 Valence Shell Electrons


Pauli Exclusion Principle

Only 2 electrons, of spin+/-1/2, can occupy the same energy state at the same point in space.


Banding of Discrete states and the Simplified Model

T=0K

EC or conduction band

EV or valence band

“Band Gap” where ‘no’ states

exist


4 electrons available for sharing (covalent bonding) in outer shell of atoms









Ec

Ev

No “Holes” valence band means no “hole” conduction is possible

No electrons in conduction band means no electron conduction

is possible

Band Occupation at Low Temperature (0 Kelvin)

For (Ethermal=kT)=0


+

Ec

Ev

Electron free to move in conduction band

“Hole” free to move in valence band

Band Occupation at Higher Temperature (T>0 Kelvin)

For (Ethermal=kT)>0


+

For (Ethermal=kT)>0

Carrier Movement Under Bias

Direction of Current Flow


Ec

Ev


“Hole” movement in valence band


+

Ec

Ev






For (Ethermal=kT)>0


+

Ec

Ev






For (Ethermal=kT)>0


+

Ec

Ev






For (Ethermal=kT)>0 QuickTime Movie


Ec

Ev

The valance band may have ~4e22 cm-3 valence electrons “participating in the bonding processes holding the crystal together.

The valance band might only have ~1e6 to 1e19 cm-3 “holes” in the valence band (missing valence electrons). Thus, it is easier to account for the influence of the holes by counting the holes directly as apposed to counting very small changes in the valence electron concentrations.

Example: If there are 1e 22 cm-3 atoms in a crystal with each atom having 4 valence electrons. What is the difference in valence electron concentration for 1e12 holes verses 1e13 cm-3 holes?

Answer: 4 x 1e22 cm-3 -1e12 cm-3 = 3.9999999999e22cm-3 verses

4 x 1e22 cm-3 -1e13 cm-3 = 3.999999999e22cm-3

For “accounting reasons” keeping track of holes is easier!

Clarification of confusing issues:“Holes” and Electrons


Clarification of confusing issues:“Holes” and ElectronsTerminology

Electrons: Sometimes referred to as conduction electrons: The electrons in the conduction band that are free to move throughout the crystal.

Holes: Missing electrons normally found in the valence band (or empty states in the valence band that would normally be filled).

If we talk about empty states in the conduction band, we DO NOT call them holes! This would be confusing. The conduction band has mostly empty states and a few electrons.

If we talk about filled states in the valence band, we DO NOT call them electrons! This would be confusing. We can call them Valence Electrons to indicate they are bond to atoms (in the valence shells of atoms). The valence band has mostly filled states and a few holes.

For the vast majority of this class we only talk about electrons (conduction band electrons) and holes (empty states in the valence band)!

Only these “particles” carry electricity. Thus, we call these “carriers”


Material Classification based on Size of Bandgap:Ease of achieving thermal population of conduction band determines

whether a material is an insulator, semiconductor, or metal

~0 Electrons in Conduction Band

~106 - 1014 cm-3 Electrons in Conduction Band without “help”

~1022 cm-3 Electrons in Conduction Band


Intrinsic Carrier Concentration

•For each electron promoted to the conduction band, one hole is left in the valence band. Thus, the number of electrons in the conduction band is equal to the number of holes in the valence band unless there is “help” to change the relative populations in each band.

•Intrinsic carrier concentration is the number of electron (=holes) per cubic centimeter populating the conduction band (or valence band) is called the intrinsic carrier concentration, ni

•ni = f(T) that increases with increasing T (more thermal energy)

ni~2e6 cm-3 for GaAs with Eg=1.42 eV, ni~1e10 cm-3 for Si with Eg=1.1 eV, ni~2e13 cm-3 for Ge with Eg=0.66 eV, ni~1e-14 cm-3 for GaN with Eg=3.4 eV

At Room Temperature (T=300 K)


Dopants

Reading:

(Cont’d) Notes


The need for more control over carrier concentration

Without “help” the total number of “carriers” (electrons and holes) is limited to 2ni.

For most materials, this is not that much, and leads to very high resistance and few useful applications.

We need to add carriers by modifying the crystal.

This process is known as “doping the crystal”.


Regarding Doping, ...


Example: P, As, Sb in Si

Extrinsic, (or doped material):Concept of a Donor “adding extra” electrons


Concept of a Donor “adding extra” electrons

11.01)4(2 2

*

2

4*

nforeVEmm

nqm

E HRo

n

oR

nelectronforBinding

Use the Hydrogen Atomic Energy levels to approximate the energy required to free an electron on a donor.

•Replace dielectric constant with that of the semiconductor

•Replace mass with that of the semiconductor

...3,2,1,arg2/2/tan,

6.13)4(2 22

4

nandechelectronqhtconsplanksmasselectronmwhere

neV

nqm

EEnergy

o

o

oHelectronHydrogen


Extrinsic, (or doped material):Concept of a Donor “adding extra” electrons


Concept of a Donor “adding extra” electrons:Band diagram equivalent view


Example: B, Al, In in Si

One less bond means the acceptor is electrically

satisfied

One less bond means the

neighboring silicon is left

with an empty state.

Extrinsic, (or doped material):Concept of an acceptor “adding extra” holes

All regions of

material are

neutrally charged.


Concept of an Acceptor“adding extra hole”:Band diagram equivalent view


Hole Movement

Empty state is located next to the Acceptor

All regions of

material are

neutrally charged.


+

Hole Movement

Another valence electron can fill the empty state located next to the Acceptor leaving behind a positively charged “hole”.


+

Hole Movement

The positively charged “hole” can move throughout the crystal (really it is the valance electrons jumping from atom to atom that creates the hole motion).


+

Hole Movement



+

Hole Movement



+

Hole Movement

Region around the acceptor has one extra electron and thus is negatively charged.

Region around the “hole” has

one less electron and

thus is positively

charged.



Summary of Important terms and symbolsBandgap Energy: Energy required to remove a valence electron and allow it to freely conduct.Intrinsic Semiconductor: A “native semiconductor” with no dopants. Electrons in the conduction band equal holes in the valence band. The concentration of electrons (=holes) is the intrinsic concentration, ni.Extrinsic Semiconductor: A doped semiconductor. Many electrical properties controlled by the dopants, not the intrinsic semiconductor.Donor: An impurity added to a semiconductor that adds an additional electron not found in the native semiconductor.Acceptor: An impurity added to a semiconductor that adds an additional hole not found in the native semiconductor.Dopant: Either an acceptor or donor.N-type material: When electron concentrations (n=number of electrons/cm3) exceed the hole concentration (normally through doping with donors).P-type material: When hole concentrations (p=number of holes/cm3) exceed the electron concentration (normally through doping with acceptors).Majority carrier: The carrier that exists in higher population (ie n if n>p, p if p>n)Minority carrier: The carrier that exists in lower population (ie n if n<p, p if p<n)Other important terms (among others): Insulator, semiconductor, metal, amorphous, polycrystalline, crystalline (or single crystal), lattice, unit cell, primitive unit cell, zincblende, lattice constant, elemental semiconductor, compound semiconductor, binary, ternary, quaternary, atomic density, Miller indices, various notations, etc...


Carrier Movement in Free Space

masselectronmechelectronicqtimetvelocityvforceF

dtdvmqEF

o

o

,arg,,,

Newtons second law


QuickTime Movie

Carrier Movement Within the Crystal•Electron is a quasi-particle that behaves as a “wave” due to quantum mechanical effects.

•The electron “wavelength” is perturbed by the crystals periodic potential.


masseffectiveelectronmechelectronicq

timetvelocityvforceFdtdvmqEF

n

n

*

*

,arg,,,

masseffectiveholemechelectronicq

timetvelocityvforceFdtdvmqEF

p

p

*

*

,arg,,,

Carrier Movement Within the Crystal

Ge and GaAs have “lighter electrons” than Si which results in faster devices


How are semiconductors

produced?


Chemical Vapor Deposition

Chemical gas sources are thermally, optically, or electrically (plasma) reacted with a surface to “leave” behind deposits with reaction byproducts pumped out of the reaction tube or vacuum chamber.


1.) Atmospheric Pressure CVD (APCVD)Advantages: High deposition rates, simple, high throughputDisadvantages: Poor uniformity, purity is less than LPCVDUsed mainly for thick oxides.

2.) Low Pressure CVD (LPCVD at ~0.2 to 20 torr)Advantages: Excellent uniformity, purityDisadvantages: Lower (but reasonable) deposition rates than APCVDUsed for polysilicon deposition, dielectric layer deposition, and doped dielectric deposition.

3.) Metal Organic CVD (MOCVD)Advantages.: Highly flexible—> can deposit semiconductors, metals, dielectricsDisadvantages: HIGHLY TOXIC!, Very expensive source material. Environmental disposal costs are high.Uses: Dominates optical (but not electronic) III-V technology, some metallization processes (W plugs and Cu)

4.) Plasma Enhanced CVDPlasmas are used to force reactions that would not be possible at low temperature.Advantages.: Uses low temperatures necessary for rear end processing. Disadvantages: Plasma damage typically results.Used for dielectrics coatings.

Four Basic CVD Reactors


Single Crystal Semiconductors (Epitaxy)

We can grow* crystalline semiconductors by raising the temperature to allow more surface migration and by using a crystalline substrate (Si, GaAs, InP wafer, etc…)===> Single crystal material mimicking the crystal structure of the layers below it.

Epitaxy

*Instead of the word deposit, we use “grow” to describe the tendency of the deposited material to mimic the crystal structure ofcrystalline substrate material.


EpitaxyImportance of lattice mismatch

The lattice constant of the epitaxially grown layer needs to be close to the lattice constant of the substrate wafer. Otherwise the bonds can not stretch far enough and dislocations will result.

DislocationStrained but unbroken bond

Strained (elongated) but unbroken bond

Strained (compressed) but unbroken bond


EpitaxyImportance of lattice mismatch

The lattice constant of the epitaxially grown layer needs to be close to the lattice constant of the substrate wafer. Otherwise the bonds can not stretch far enough and dislocations will result.


Primarily used for II-VI, and III-V semiconductors, special metallic oxides and metals.

Metal Organic Chemical Vapor Deposition (MOCVD)•Many materials that we wish to deposit have very low vapor pressures and thus are difficult to transport via gases.•One solution is to chemically attach the metal (Ga, Al, Cu, etc…) to an organic compound that has a very high vapor pressure. Organic compounds often have very high vapor pressure (for example, alcohol has a strong odor).•The organic-metal bond is very weak and can be broken via thermal means on wafer, depositing the metal with the high vapor pressure organic being pumped away.•Care must be taken to insure little of the organic byproducts are incorporated. Carbon contamination and unintentional Hydrogen incorporation are sometimes a problem.

Human Hazard: As the human body absorbs organic compounds very easily, the metal organics are very easily absorbed by humans. Once in the body, the weak metal-organic bond is easily broken, thus, poisoning the body with heavy metals that often can not be easily removed by normal bodily functions. In extreme cases, blood transfusion is the only solution (if caught in time).

MOCVD


Commercial Thomas Swan® MOCVD


Molecular Beam Epitaxy (MBE)Dominates III-V electronic market and strong competitor in upper end LASER marketOffers the highest purity material (due to UHV conditions) and the best layer control (almost any fraction of an atomic layer can be deposited and layers can be sequenced one layer at a time (for example Ga then As then Ga etc…).

MBE

•In an UHV chamber, ultra high purity materials are evaporated.•Because of the very low pressure, the mean free path is very long (can be hundreds of meters). Thus, the evaporated material travels in a straight line (a molecular beam) toward a hot substrate.•Once on the substrate, the atom or molecule moves around until it finds an atomic site to chemically bond to.•Shutters can be used to turn the beam flux on and off•The flux of atoms/molecules is controlled by the temperature of the “effusion cell” (evaporation source).


MBE

Effusion Furnaces

Partially disassembled MBE system for clarity

RHEED Gun

Gas Source (oxygen)

Shutter mechanism


Commercial Veeco® MBE


Crystalline Order

Water Molecules, H2O, forming “Snowflakes”Atoms forming a “Semiconductor”

Need two volunteers… (demo on how a crystal forms naturally due to

repulsive electronic bonds)


Compound Semiconductors allow us to perform “Bandgap Engineering” by changing the energy bandgap as a function of position. This allows the electrons to see “engineered potentials” that “guide” electrons/holes in specific directions or even “trap” them in specific regions of devices designed by the electrical engineer.

Example: Consider the simplified band diagram of a GaN/ Ga0.75In0.25N/ GaN LED structure. Electrons and holes can be “localized” (trapped) in a very small region – enhancing the chance they will interact (recombine). This is great for light emitters!

Heterojunction Materials

Econduction

Evalence

Light


How do we produce these Energy Engineered Structures and Devices?

Epitaxial Semiconductor and Dielectric deposition Techniques:•“Epitaxial” is derived from the Greek word for skin, more specifically “thin skin”. Thin layers of materials are deposited on a substrate

•Temperature and substrate determines the physical structure of the deposited films:

•Low Temperatures or non-crystalline substrate:

•Materials end up with amorphous or polycrystalline materials

•High Temperature AND Crystalline substrate

•Need to have an existing crystalline wafer so as to “seed” the crystallization process.

•Films that retain the substrates basic crystal structure are “Epitaxial”


Single Crystal Semiconductors (Epitaxy)

We can grow* crystalline semiconductors by raising the temperature to allow more atom surface migration (movement of atoms due to thermal energy) and by using a crystalline substrate (Si, GaAs, InP wafer, etc…) to act as a “template” or crystalline pattern. This results in a single crystal material mimicking the crystal structure of the layers below it.

Epitaxy

*Instead of the word deposit, we use “grow” to describe the tendency of the deposited material to mimic the crystal structure ofcrystalline substrate material.


Advanced Semiconductor Technology Facility

Dr. W. Alan Doolittle

Contact Information:Phone and Fax: 404-894-9884Email: [email protected]: School of Electrical and Computer EngineeringGeorgia Institute of Technology777 Atlantic Dr.Atlanta, GA 30332-0250


Molecular Beam Epitaxy (MBE)Dominates III-V electronic market and strong competitor in upper end LASER marketOffers the highest purity material (due to UHV conditions) and the best layer control (almost any fraction of an atomic layer can be deposited and layers can be sequenced one layer at a time (for example Ga then As then Ga etc…).

Molecular Beam Epitaxy (MBE)

•In an UHV chamber, ultra high purity materials are evaporated.•Because of the very low pressure, the mean free path is very long (can be hundreds of meters). Thus, the evaporated material travels in a straight line (a molecular beam) toward a hot substrate resulting in highly efficient raw materials usage.•Once on the substrate, the atom or molecule moves around until it finds an atomic site to chemically bond to.•Shutters can be used to turn the beam flux on and off•The flux of atoms/molecules is controlled by the temperature of the “effusion cell” (evaporation source).






How do we create Bandgap Engineered Structures? Epitaxy

•Repeating a crystalline structure by the atom by atom addition.

•Chemistry controls the epitaxy to insure that, for example, Ga bonds only to N and not Ga-Ga or N-N bonds*.

*A small number of “antisite” defects (Ga-Ga or N-N bonds) actually do form but are typically in the parts per trillion concentration.

























GaN

GaN

GaN

GaN

AlN

AlGaN

AlGaN

GaN

Ec Ev


Ec

Ev

E=-qVArbitrary Reference Energy

Kinetic Energy

Potential Energy

Engineered Energy Behavior in Compound Semiconductors

The potential distributions we will use in this class are all possible/common in device structures. Some may represent “grown in potentials” (quantum wells, etc...) or naturally occurring potentials (parabolic potentials often occur in nature – lattice vibrations for example) including periodic potentials such as lattice atoms.


Production of Solar Cell Grade Semiconductors

First solid Thin-film selenium solar cell demonstrated by Fritts in 1883.

Early Grondahl-Geiger copper-cuprous oxide photovoltaic cell (circa 1927).

Structure of the first “modern” Si solar cell. 6% efficiency (~15x improvement at the time).Chapin, D.M., Fuller, C.S. and Pearson, G.L., "A New Silicon P-N Junction Photocell for Converting Solar Radiation into Electrical Power", Journal of Applied Physics, Vol. 25, pp. 676-677, 1954.



Descriptor Symbol Grain Size Common Growth Techniques

Single crystal sc-Si >10cm Czochralski (CZ) float zone (FZ)

Multicrystalline mc-Si 1mm-10cm Cast, sheet, ribbon

Polycrystalline pc-Si 1µm-1mm Chemical-vapour deposition

Microcrystalline µc-Si <1µm Plasma deposition


Crystal Growth: How do we get “Single Crystalline Material”?The vast majority of crystalline silicon produced is grown by the Czochralski growth method. In this method, a single crystal seed wafer is brought into contact with a liquid Silicon charge held in a crucible (typically SiO2 but may have a lining of silicon-nitride or other material). The seed is pulled out of the melt, allowing Si to solidify. The solidified material bonds to the seed crystal in the same atomic pattern as the seed crystal.


Production of Solar Cell Grade SemiconductorsFloat Zone Refinement for Ultra-High grade Silicon






Production of Solar Cell Grade SemiconductorsEdge Film Grown


Production of Solar Cell Grade SemiconductorsDendritic Web Silicon


Production of Solar Cell Grade SemiconductorsHigh performance Laser Grooved, plated and random

surface textured Si Solar Cells


Production of Solar Cell Grade SemiconductorsHigh performance Inverted Pyramid Si Solar Cells with

minimal metal-semiconductor contact area


Production of Solar Cell Grade SemiconductorsHigh performance Back Contact Si Solar Cells with minimal shadowing and minimal metal-semiconductor contact area

Documents

Lecture 3b— Introduction to semiconductors